A diffractive optical element (DOE) is an optical element performing optical functions of such as focusing, color-separation (wavelength demultiplexing), refraction, reflection and imaging based on the optical diffraction principle. A typical diffractive optical element has a micro surface-relief profile which enables the phase of an incident light beam through the diffractive optical element to change (namely, phase modulation) so as to obtain a desired or designated output light distribution. A modulation amount of the phase of the incident light beam caused by the microrelief profile on the surface of the diffractive optical element is related to its thickness, that is, modulation thickness. Therefore, a core issue in designing the diffractive optical element is to determine a modulation thickness at each position (sampling point) of its surface.
There are many documents studying the design of the diffractive optical element. For example, references are listed in the following:
[1] G. Z. Yang, B. Y. Gu, et al.“Iterative optimization approach for the design of diffractive phase elements simultaneously implementing several optical functions,”J. Opt. Soc. Am. A Vol. 11, 1632-1640 (1994).
[2] B. Z. Dong, G. Q. Zhang, G. Z. Yang, B. Y. Gu, et al. “Design and fabrication of a diffractive phase element for wavelength demultiplexing and spatial focusing simultaneously,” Appl. Opt. Vol. 35, 6859-6864 (1996).
[3] B. Z. Dong, G. Z. Yang, B. Y. Gu, et al. “Diffractive phase elements that implement wavelength demultiplexing and spatial annular focusing simultaneously,” J. Opt. Soc. Am. A Vol. 14, 44-48 (1997).
[4] B. Z. Dong, R. Liu, G. Z. Yang and B. Y. Gu, “Design of diffractive phase elements that generate monochromatic or color point ad ring patterns,” J. Opt. Soc. Am. A Vol. 15, 480-486 (1998).
[5] B. Y. Gu, G. Z. Yang, B. Z. Dong, et al., “Diffractive-phase-element design that implements several optical functions,”Appl. Opt. 34, 2564-2570 (1995).
[6] G. Z. Yang, B. Z. Dong, B. Y. Gu, et al., “Gerchberg-Saxton and Yang-Gu algorithms for phase retrieval in a nonunitary transform system: a comparison,” Appl. Opt. 32, 209-218 (1994).
In the above-listed and other unlisted relevant references, Yang-Gu algorithm is adopted to solve a phase modulation amount for the incident light beam or the modulation thickness of the diffractive optical element, and both color-separating and focusing functions for the incident light beam having a plurality of wavelength components can be achieved simultaneously by means of a single diffractive optical element. However, the diffractive optical element designed according to the existing methods only has a diffraction efficiency of 10%-20%, thereby the diffractive optical element is limited in certain applications. It should be noted that the diffraction efficiency of the diffractive optical element here refers to a proportion of energy of output light in a focus area to incident light energy.
Another aspect involved in the present application is related to solar cells. Energy issue is an important topic of common concern for all countries in today's world, and pollution-free solar energy is a strategic resource which all countries are striving to utilize. A solar cell is a device converting solar energy to electrical energy. A basic principle of solar cells goes as follows: when sunlight irradiates photovoltaic materials in the solar cell, e.g., semiconductor materials, the semiconductor materials convert light energy into electrical energy because of a photovoltaic effect after absorbing the sunlight irradiation. Since different semiconductor materials have different bandgap structures, a semiconductor material can have a higher photoelectric conversion efficiency only around a respective wavelength component of the incident light beam corresponding to the bandgap energy thereof. Provided that semiconductors having different bandgaps can be used to absorb light around respective wavelength components corresponding to the bandgap energies, this will substantially improve the photoelectric conversion efficiency of solar energy. Studies also indicate that if sunlight is focused, the conversion efficiency of solar energy will not deteriorate due to the increase of light intensity, but improve to some extent. In the case that the sunlight is focused, a lot of expensive materials may be saved, and a less cell area may be used to convert more light energy.
In view of the above idea of utilizing solar energy, currently there are chiefly two classes of study schemes for solar cells in the world, namely, series (also known as “cascading”) or parallel (also known as “lateral”) mode. In the “series” configuration, different semiconductor materials grow in turn from down to up in a vertical direction and their bandgap energies increases gradually. An incident light beam, after being focused by an optical system, passes through all layers of materials. A shortest wavelength component (with a maximum energy) in the incident light beam is absorbed by the uppermost layer of semiconductor material. Along with the increase of the wavelength, the incident light beam is absorbed in turn by the lower layers of semiconductor materials. As such, a higher conversion efficiency can be achieved. A drawback of this scheme lies in that an interface between different layers of semiconductors needs to grow by virtue of technologies such as molecular beam epitaxy, which is very difficult to be controlled precisely and has great limitation of choices of materials so as to cause the manufacture cost very high. The parallel configuration refers to performing the color-separation while focusing the sunlight, so that sunlight having different wavelength components is focused to different positions, each of which a semiconductor material with a bandgap energy approximate to photon energy of the corresponding wavelength component is placed at, thereby substantially improving the photoelectric conversion efficiency of the solar energy. Currently, there are mainly two existing schemes for achieving the parallel configuration. The first scheme is to perform color-separation for the sunlight by means of a dichroic mirror so as to separate the sunlight into two waveband components, a longwave component and a shortwave component. To achieve a higher efficiency of the color-separation, the dichroic mirror usually needs to be coated with more than ten layers or even dozes of layers of films, which is technically very difficult. The second scheme is to use a lens-prism combination to perform color-separation, and this scheme makes the optical device bulky. A drawback of the existing parallel configurations lies in that the cost of the optical system is very high.
If the thickness of a diffractive optical element can be controlled within a certain range, it may be fabricated and copied in batches by modern photolithography technology so that the cost of the optical system may be greatly reduced. If the diffractive optical element has functions of color-separating and focusing, the cost of the whole color-separating and focusing photovoltaic system may be substantially cut.
However, as stated in the above description of the diffractive optical element, a diffraction efficiency of the current diffractive optical element for color-separating and focusing is only 10%-20%, such diffraction efficiency is obviously not high enough to utilization of the solar energy and hinders application of the color-separating and focusing diffractive optical element to the solar cell.